Individuals suffering from kidney failure require regular blood dialysis treatment. Such dialysis treatment requires the removal of blood from the individual and the cycling of the blood through a dialysis machine that performs the function of the failed kidney. After processing, the blood is then returned to the individual. Typically, the procedure is performed three times a week over many years and each treatment requires the insertion of a dialysis needle for withdrawal and returning of the blood. Furthermore, dialysis requires a relatively rapid blood flow rate so it is necessary for the dialysis needles to be quite large.
The problem with regular dialysis treatment of this type is that the natural blood vessels of patients are of insufficient strength to withstand collapse from frequent puncturing with large bore needles and allow sufficient flow rate through the vessel to achieve an acceptable rate of dialysis. One solution to this problem is to create a site within a patient, in a surgical procedure, by bridging an artery and a vein under the skin on the inside of the forearm or the upper thigh with a synthetic graft. This surgical procedure provides an easily accessible blood vessel into which the dialysis needles are inserted. It is instead possible to form an autograft by suturing together an artery and a vein but, while this creates an easily accessible site, the auto graft is still susceptible to scarring and collapse as with any natural blood vessels.
Where a synthetic graft (which will hereinafter be termed an “access graft” or “shunt grant”) is implanted, it is desirable that the graft has self-sealing properties because otherwise blood can leak from apertures left in the access graft following puncturing by the dialysis needles, particularly in the weeks immediately following implantation of the access graft and prior to full healing around the graft (see Daugirdas et al Handbook of Dialysis).
Many different types of access grafts with self-sealing properties are known in the art. For example, in U.S. Pat. No. 4,619,641 there is disclosed an access graft which comprises a coaxial double lumen tube. The inner and outer tubes are made from, for example, Teflon PTFE or Dacron synthetic polyester fibre. The space between the outer tube and the inner tube is filled with a biocompatible polymer such as silicone rubber sealant.
WO01/28456 discloses a laminated self-sealing vascular access graft. The graft comprises an inner layer, an intermediate layer concentrically surrounding the inner layer and an outer layer concentrically surrounding both the inner and intermediate layers. The inner layer is made from expanded PTFE. The intermediate layer comprises alternating regions of materials of different densities, including a low density material such as PTFE “cotton”.
Other examples of self-sealing access grafts include grafts composed of heparin-bonded polycarbonate (see Hazinedaroglu et al Transplantation Proceedings 2004; 36(9): 2599 to 2602) or polyurethane (see Wearn et al Asian Cardiovascular and Thoracic Annals 2003; 11: 314-318).
The problem with such synthetic access grafts is that at the artery/vein junction, the internal lumen of the graft can narrow over time. Furthermore, repeated puncturing of even synthetic, self-sealing access grafts during the dialysis process can result in internal scarring which leads to reduced flow and eventual occlusion. Moreover, blood flowing through an access graft is generally very turbulent especially when a dialysis needle is inserted into the graft, during dialysis. The turbulent flow results in fibrils forming on the interior of the access grafts, again leading to reduced flow and even occlusion of the access graft.
The present invention seeks to alleviate one or more of the above problems.